If you or someone you cook for has food allergies, you need to find substitutes for many common ingredients, such as wheat flour and corn starch, depending on the source of the allergy. Certain ingredients, such as xanthan gum and potato starch, can perform the roles commonly taken on by other ingredients. Both are common in gluten-free baking and cooking, but they have slightly different characteristics and uses.

A common ingredient in both gluten-free recipes and processed foods, xanthan gum is the product of fermenting glucose with Xanthomonas campestris, a type of bacteria. Xanthan gum plays several roles in cooking. It acts as an emulsifier, which is why it's often used in bottled salad dressings. In a recipe for gluten-free bread, xanthan gum helps replace the gluten. It interacts with the starches in the recipe, trapping air and giving the bread some chewiness.

Potato starch is quite different from xanthan gum, both in terms of where it comes from and the role it plays in cooking. The starch is made up of long amylose chains, or straight chains of glucose. Potato starch is commonly used to thicken sauces and to give a lighter texture to gluten-free baked goods. Unlike other types of starch, such as cornstarch, potato starch is clear, has a moderate flavor and thickens at a relatively low temperature, between 136 and 150 degrees Fahrenheit. The downside is that the starch breaks down more quickly and can't withstand long cooking times.

You can use either xanthan gum or potato starch to replace wheat flour called for in a recipe for gravy, soup or stew. Since both ingredients have more thickening power than wheat flour or starch, you can use considerably less in a recipe. Use 1 1/2 teaspoons of potato starch to replace each tablespoon of wheat flour called for in a recipe, for example. You can use even less xanthan gum to replace wheat flour. Substitute 1 teaspoon xanthan gum for each tablespoon of wheat flour.

Although xanthan gum plays more roles in the kitchen than potato starch, a couple of factors might give you pause. For one thing, xanthan gum is considerably more expensive than potato starch. An ounce of xanthan gum can cost more than $1 while an ounce of potato starch can cost less than 30 cents, as of 2013. Xanthan gum can also cause gastrointestinal problems or bloating in some people, because it contains 7 grams of fiber per tablespoon.